Answered by Shaykh Amjad Rasheed
A man accepts Islam and then marries a Muslim woman. This recent convert’s mother remains non-Muslim and obtains a divorce from her husband and then gets married to another non-Muslim. Is this new non-Muslim husband considered a mahram to her son’s Muslim wife? What parts of her body can the Muslim wife uncover in front of this non-Muslim man?
In the Name of Allah, Most Merciful and Compassionate
The stepfather (lit. “mother’s husband”) is not considered mahram to the wife of his wife’s son (who is referred to by the fuqaha as a rabib), regardless of whether the stepfather is Muslim or non-Muslim. The great scholar al-Bajuri says in his supercommentary on Ibn Qasim (Hashiya ‘ala Ibn Qasim): “The wife of the stepson — i.e. the son of one’s wife — is not unmarriageable.” (Hashiya al-Bajuri, 2.117) It is therefore obligatory for the Muslim wife to observe the rules of hijab in front of her husband’s stepfather.
(Shaykh Amjad Rasheed has spent several years studying Shafi’i fiqh in Jordan, Syria, and Hadramawt. He currently lives and teaches in Amman. The Shafii Fiqh Forum translates questions it receives and sends them to him to answer and then translates his Arabic answers and posts them on the list.)
 A mahram is an unmarriageable close relative in front of whom one does not have to observe the rules of hijab.